Field of the Invention
The present invention relates to a membrane humidifier for a fuel cell.
Description of Related Art
A fuel cell includes a fuel cell stack to generate electric energy, a fuel supply system to supply fuel (e.g., hydrogen) to the fuel cell stack, an air supply system to supply oxygen, which is contained in the air and serves as an oxidizer required for an electrochemical reaction, to the fuel cell stack, and a heat and water management system to control the operation temperature of the fuel cell stack.
Referring to FIG. 1, the air supply system of the fuel cell includes an air blower 30 to supply external air to a cathode of the fuel cell stack 10, and a membrane humidifier 20 to humidify the air supplied from the air blower 30 and then supply the humidified air to the cathode of the fuel cell stack 10.
In the case of a polymer electrolyte membrane fuel cell, water is required for the operation of the fuel cell, and a humidifier for humidifying the air is typically used. In order to humidify the air, various methods may be used such as, for example, a bubbler, injection, and an absorbent may be used. However, since the size of a fuel cell used for a vehicle is significantly constrained, fuel cell vehicles use a membrane humidifier that has a relatively small volume and requires no power.
FIGS. 2 and 3 illustrate a conventional membrane humidifier. Such a membrane humidifier includes a housing 100, a hollow fiber membrane module 200, and manifolds 106 and 108. The housing 100 includes an inlet 102 formed on one side end thereof to receive external air (e.g., dry air) from the air blower 30 and an outlet 104 formed on the other side end thereof to discharge humidified air toward the fuel cell stack 10. The manifold 106 is formed on the top surface of the housing 100 proximate to outlet 104 to receive wet air discharged from the stack, and the manifold 108 is formed on the bottom surface of the housing 100 proximate to inlet 102 to discharge the wet air, obtained by humidifying the air from the air blower, to the outside.
Furthermore, the hollow fiber membrane module 200 includes a bundle of hollow fiber membranes 202 housed therein, and the housing 100 having the manifolds 106 and 108 formed therein is coupled to both ends of the hollow fiber membrane module 200 so as to surround the hollow fiber membrane module 200.
The operation of the conventional membrane humidifier having the above-described structure will be described briefly as follows.
As shown in FIG. 3, when wet air discharged from the fuel cell stack is supplied to the inside of the hollow fiber module 200 through the manifold 106 of the housing 100, water contained in the wet air is separated by the capillary action of the respective hollow fiber membranes 202 housed in the housing 100. The separated water is condensed while passing through capillary tubes of the hollow fiber membranes 202, and moved into the hollow fiber membranes 202. At this time, the wet air from which the water is separated is moved to the outside of the hollow fiber membranes 202 and discharged as external air through the manifold 108 of the housing 100.
The external air (e.g., dry air) supplied from the air blower 30 through the inlet 102 of the housing 100 is moved while passing through the hollow fiber membranes 202. At this time, since the water separated from the wet air is already moved into the hollow fiber membranes 202, the dry air is humidified by the water. The humidified air is supplied toward the fuel cell stack 10 through the outlet 104.
Hereinafter, a conventional method for manufacturing the hollow fiber membrane module of the membrane humidifier having the above-described structure and operation will be described as follows.
First, a case is manufactured, and a desired number of hollow fiber membranes are housed therein. Then, a polymer material is injected into both ends of the hollow fiber membranes to fix the hollow fiber membranes to the case. This process is referred to as “potting” or a “potting process.” The polymer material used for potting may include a urethane-based resin material, and the potting process may include a dipping process and a centrifugal molding process. The dipping process takes advantage of the use of gravity, as illustrated in FIG. 4, and is performed as follows. First, a case 204 having a bundle of hollow fiber membranes 202 housed therein is inserted into a resin injection device. Then, when a polymer material is injected through an inlet of the resin injection device, the injected polymer material fixes the bundle of hollow fiber membranes while permeating between the densely-housed hollow fiber membranes due to gravity.
FIGS. 5A to 5D are diagrams illustrating such a potting process. The potting process includes preparing a case as in FIG. 5A, putting a potting cap 300 on as in FIG. 5B, inserting the hollow fiber membranes 202 as in FIG. 5C, and injecting resin 310 as in FIG. 5D. As illustrated in FIG. 6, the potting material is injected into the case through a hole 110.
After the potting process, the polymer material is dried, and an end of the potted portion is partially cut by a cutting device. Then, a hollow fiber membrane module having a cross-sectional structure as illustrated in FIG. 6 may be obtained.
In accordance with the conventional method, the potting material repetitively contracts and expands according to temperature changes. As a result of the repetitive contraction and expansion of the potting material, a gap 320 may be formed between the potting material 310 and the case 204 after a predetermined time passes, as illustrated in FIGS. 7A and 7B. Because of the gap 320, air tightness may not be maintained. FIG. 8 is a photograph showing a state in which a gap is formed between the potting material and the case.
In the conventional method, an air flow within the humidifier may apply stress to the polymer hollow fiber membranes. In this case, a hollow fiber membrane may be cut at a portion where the hollow fiber membrane is contacted with the bottom of the potting material 310.
Furthermore, when the polymer hollow fiber membrane is potted by a potting material, the potted portion of the hollow fiber membrane does not function as a humidifying membrane. That is, as the potted portion is formed to a large thickness, the humidification performance of the humidifier decreases. Accordingly, there is a need for a membrane-based humidifier that does not deform under physical stress (e.g., temperature induced expansion/contraction, air pressure, etc.), and therefore does not form a gap, as well as for a membrane that maintains humidification performance even as a thick membrane.